1. Field of the Invention
The present invention is directed to a method and apparatus for use in the photolithographic arts, particularly the semiconductor fabrication arts, which ensures rapid and accurate acquisition, via a pattern recognition technique, of a target indicia photolithographically reproduced on a test wafer. Target acquisition is, generally speaking, a subset of the overall process of CD (critical dimension) measurement of circuitry patterns on semiconductor-based dies, although the invention may also find utility in other areas.
2. Description of the Related Art
In the fabrication of semiconductor devices, photolithographic masks are used to transfer circuitry patterns to silicon wafers in the creation of integrated circuits. In general, in the production of semiconductor circuit devices, a series of such masks are employed in a preset sequence. Each photolithographic mask includes an intricate pattern of CAD-generated geometric patterns corresponding to the circuit components to be integrated onto the wafer. Each mask in the series is used to transfer its corresponding pattern onto a photosensitive layer (photoresist) which has been previously deposited on the silicon wafer. The transfer of the mask pattern onto the photosensitive layer or photoresist is currently performed by an optical exposure tool, which directs light or radiation through the mask to the photoresist.
Fabrication of the photolithographic mask follows a set of predetermined design rules which are set by processing and design limitations. For example, these design rules define the space tolerance between devices or interconnecting lines, and the width of the lines themselves, to ensure that the devices or lines do not overlap or interact with one another in undesirable ways. The design rule limitation is referred to within the industry as the xe2x80x9cCDxe2x80x9d (critical dimension). The critical dimension of a circuit is defined as the smallest width of a line or the smallest space between two lines which is to be permitted in the fabrication of the chip. More often than not, the CD is determined by the resolution limit of the exposure equipment. Presently, the CD for most applications is on the order of a fraction of a micron. Because of the extremely small scale of the CD, the instrument of choice for measurement and inspection is a scanning electron microscope (SEM).
When new masks are produced, or after any change in the fabrication recipe, it is customary to form a so-called focus exposure matrix (FEM) on a test wafer in order to obtain the best exposure/focus combination for the mask, e.g., the combination of focus and exposure which results in the best resolution on the wafer, in keeping with the required CD. This is typically done by, for example, sequentially exposing a series of areas of the wafer with the pattern of the mask, while exposure and focus values are incrementally changed from one exposure location to the next. After exposure of the wafer in this fashion, one can examine the individual exposure sites, for example, to check the CD, and obtain a plot of exposure v. focus or focus v. CD and determine the area of best performance from the resulting curves. Specifically, a test wafer is exposed in a stepper while the focus is varied along one axis and the exposure is varied along the other. Thus, a matrix of images is obtained on the exposed wafer, wherein each exposure site or die has a different focus-exposure setting. Selected CDs (at various locations) in each die are measured to select the best exposure-focus setting for the particular mask.
The general procedure for determining the CD in a test wafer is as follows. First, an alignment target (which is not part of the circuitry) is included on the mask, typically at an area which will not include circuit patterns. During exposure, an image of the alignment target is transferred onto each of the dies. When the test wafer is developed and loaded into the CD measurement machine (typically a CD SEM) the operator first causes the system to acquire the alignment target of the central or reference die of the wafer. The image of this alignment target is stored in memory for reference. The operator then acquires an appropriate area for CD measurement, and designates that area to the CD machine. The machine automatically calculates a vector from the center of the alignment target to the center of the designated area. This procedure is repeated for each area which the operator wishes to measure.
The foregoing procedure can be performed in what might be designated as a xe2x80x9cteaching modexe2x80x9d of the CD SEM. Once all of the data has been input and the vectors calculated, the CD system may then be enabled for automated CD measurement as described below.
When the developed wafer is properly loaded into the CD machine, the machine moves to the first die to be inspected and searches for the alignment target using a pattern recognition (PR) algorithm, using the aforementioned stored alignment target as a reference. When a high PR score is achieved, it is considered that the alignment target has been acquired. Using the stored vector, the CD machine then moves to the designated CD measurement site and acquires an image for CD measurement, which is then performed. Following this procedure, which may be duplicated for other locations on the die, the CD machine then goes to the next die and again using the PR algorithm searches for the alignment target using the stored target as a reference. Once a high PR score is achieved, the CD machine goes to the CD measurement site using the stored vector. This process is repeated until all of the designated dies have been measured.
At least two problems exist with the foregoing technique. In particular, since the exposure-focus settings are by definition different for each die, the alignment target on each die appears somewhat different. Thus, it is difficult to consistently achieve a high PR score when comparing a specific target on a specific die to the stored target. In general, the correlation between the stored target and the target on any particular die will be a function of the spatial or exposure-sequence distance from the die containing the reference target to the die currently under measurement. In other words, lower PR scores would normally be expected at greater distances from the reference die. Accordingly, instances may occur where the CD SEM is unable to acquire the target, particularly at distant dies. Moreover, even if the target is acquired, since the target may be misshapen as compared to the stored target, the CD machine will not always align, i.e., center on, the same point of the alignment target. In consequence, when the CD machine moves to the measurement site using the stored vector, it will not always arrive at the correct measurement location on each die. Thus, the CD of different dies would actually be measured at locations different from that designated by the operator, possibly rendering useless the entire measurement procedure.
In order to solve the aforementioned problems occurring in the prior art, the present invention employs a method whereby the stored image of the alignment target is continuously updated, in the manner such that the stored target image always closely approximates the next target to be acquired. Thus, according to the invention, difficulties in recognizing and centering on the target are minimized, and CD measurements of much higher reliability can be effected.
According to another feature of the invention, the CAD design of the target is fed to a virtual stepper software which simulates the changes the target would undergo under the different focus-exposure conditions during the FEM construction, and outputs a series of simulated target images corresponding to the FEM. These images are used in three manners. First, each image is used as a check to verify the accuracy of the acquisition of each target. Second, each image is used as a back-up in case the comparison to the preceding target fails. Third, the simulated images are used to train the PR algorithm to anticipate the changes in the target""s appearance as the system moves from die to die.
In a further implementation of the invention, a xe2x80x9cgolden targetxe2x80x9d image is acquired and stored. Then, each time a target is acquired using the FEM, a correlation score is obtained for the correlation between the acquired target and the golden target. The collection of correlation scores can be plotted together with the FEM plot, and/or is stored as a look-up table. Thereafter, during the process of CD measurement for line monitoring, if the CD shows an undesired variation, the correlation score look-up table can be consulted to determine whether the cause for the deviation is focus or exposure drift.
The above and other features and advantages will be further described below with reference to preferred embodiments thereof.